The liquid dye laser is currently used as a source of laser excitation or amplification in applications of laser enrichment, such as is shown, for example, in U.S. Pat. No. 3,944,947. In such applications, there are at least three principal objectives for the laser oscillator or amplifier, namely high energy in each laser pulse, high repetition rate, and an output beam of laser radiation of high optical quality and low divergence. In these applications, output powers of several hundreds of watts at pulse rates of several hundreds of pulses per second in combination with an output beam as close to diffraction limited as possible are desired goals. One of the important advances in laser systems in achieving these objectives has been the transverse pump laser, as shown in U.S. Pat. No. 3,740,665, wherein the optical axis is transverse to the flow direction of the fluent laser material, typically a liquid dye solution. This permits a rapid replenishment of dye into the region of the optical axis to replace expended dye, increasing both power and repetition rate.
To some extent, both power and repetition rate can be augmented by increasing the level of applied excitation to the optical axis and by increasing the flow velocity of the fluent laser material. Excessive heating and breakdown in fluid dynamic flow characteristics are limiting factors here as well as turbulence due to heating effects. In addition, the variation in refractive index throughout the fluent laser material produced by temperature variations greatly degrade the beam quality.
One source of such temperature gradients and turbulence is the heating of flow channel walls upstream of the lasing area by absorption of stray fluorescent radiation produced by the lasing of the liquid dye. For flow channels in which the flow upstream of the laser region is not strictly laminar, temperature and the associated refractive index gradients generated as the liquid moves past the heated channel wall may appear in the laser region during subsequent pulses. This reduces laser output energy, which in turn increases the amount of fluorescence available to cause further channel wall heating, reducing the laser outputs still further.